1. Field of the Invention
The present invention relates to a process for production of expansion-molded articles in a mold of linear low density polyethylene resins (hereafter referred to as L-LDPE) which are employed as thermal insulators, shock absorbing packaging materials, reusable containers, core materials for bumpers, etc.
2. Description of the Prior Art
Expansion-molded articles in a mold of L-LDPE are excellent in resistance to chemicals, resistance to heat, strain recovery after compression, etc. as compared to expansion-molded articles in a mold of polystyrene. Further the expansion-molded articles of L-LDPE are also excellent in resistant to heat, compressive strength, tensile strength, etc. even when compared with expansion-molded articles of low density polyethylene so that they have been used as shock absorbing packaging materials, reusable containers, core materials for bumpers, etc. To produce such expansion-molded articles of L-LDPE, etc., there are known the following methods;
(A) Method which comprises treating pre-expanded beads of polyolefin with an inorganic gas under pressure to impregnate the beads with the inorganic gas, gradually releasing the pressure, filling with the beads a mold capable of closing but incapable of sealing while the inner pressure of the beads is kept at 1.18 atms. or more and thermally fusing with steam, etc. to prepare a molded article in the shape of the mold (U.S. Pat. No. 3,953,558). PA1 (B) Method which comprises filling with pre-expanded beads of polyolefin a mold capable of closing but incapable of sealing, thermally fusing with steam or the like, withdrawing the molded article from the mold and, thermally aging while the volume of the molded article is 70 to 110% of the volume of the mold to prepare the molded article in the shape of the mold (U.S. Pat. No. 4,631,159). PA1 (C) Method which comprises compressing pre-expanded beads of cross-linked polyolefin to 80% or less of an apparent bulk volume of the raw beads by gaseous pressure, filling with the compressed beads a molding mold and thermally fusing to give a molded article in the shape of the mold (Published Examined Japanese Patent Application No. 33996/1978).
However, Method (A) described above involves a defect that initial investment becomes large because installation for the compressive treatment with inorganic gas is of a large scale and Method (B) has also a defect that products having a complicated shape are inferior in visual appearance such as sink mark, surface property, etc. Further Method (C) is not sufficiently satisfactory in surface property and dimensional accuracy.